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Tuesday, February 19, 2019

Notes On Mediumwave Loops And Wire Antennas

The following comments pertain to mediumwave reception.

I've constructed many loops here, both passive-tuned and hard wired for the DSP radios.

A question arose recently asking what is the equivalent air loop size to a 200mm (8 inch) length ferrite? Speaking from experience, I'll submit an educated guess and say the rough passive-tuned loop equivalent is an approximate square loop of 8-9 inches.

I've built smaller loops, on the order of 6 inches. They produce a signal strength about equivalent to the existing ferrite in a Tecsun PL-380, which is just shy of 4 inches long. The problem I've had with the really small loops of that size is that the nulling is pretty poor. The figure-8 pattern is not well-defined.

HARD-WIRED LOOPS

I have a hacked Tecsun PL-380 with the ferrite removed, and also an Eton Traveler 3 with the ferrite removed. I soldered wire leads in place and brought them out the tops of the radios for testing hard wired loops. I use micro-clips for all connections here for testing.

Two loops are currently in use here: an 8 inch and an 18 inch. Both are square loops, close-flat-wound with insulated telephone wire, about 24 gauge solid. The 8 inch has 26 turns and the 18 inch has 12 turns. If you wind a 12 inch loop, use 16 turns.

Both the 8 inch and 18 inch give good hard-wired results. Of course the 18 inch blows the doors off the 8 inch, and is the better nuller. You don't need a 5:1 step up transformer for impedance matching. Use the fully-wound loop. Inductance for both these loops is ballpark around 240 µH. I've found that is a good value to shoot for.

I will say, be careful if hard-wiring directly to the input of the DSP radios. They are very sensitive to static. I haven't blown one out yet, but I've come close. Any static or spike on the line when clipping to the chip will send it into desense for a few minutes, even if off. It eventually recovers, at least mine have so far.

At night, the PL-380 handles an 18 inch hard-wired loop pretty well without overloading. Not so much the Eton Traveler 3. Its sensitivity is a touch better than the PL-380, and otherwise will occasionally overload. A 12 inch loop might be a better bet for the Traveler 3. You shouldn't have any overload trouble with the 8 inch loop.

PASSIVE-TUNED LOOPS

Both 8 inch and 18 inch loops can be made passive and tuned with a 365 pF variable capacitor very nicely. The 18 inch ranges out nicely from 530-1700 KHz without further attention. The 8 inch only tunes to about 1430 KHz, so I jumper a clip wire across a few turns to get to 1700 KHz.

The advantage of the passive-tuned loop is that you can reduce the coupling by moving the loop a bit farther away from the radio, thus reducing the signal overload problem.

As others have stated, coupling a passive-tuned loop to a modern DSP chipped radio can be finicky because the radio gets de-tuned by the loop, then it re-tunes, then the loop is upset again. I have best results by coupling the passive loop off the end of the radio's ferrite. The tuning interaction is less. As so:

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O  --RADIO FERRITE--
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These DSP radios will start to overload much above 80 or 90 dBµ on the RSSI meter. Incidentally, the Eton Traveler 3 measures to 99 dBµ but the PL-380 only measures to 63 dBµ.

Remember, an RSSI reading of 34 dBµ is the equivalent of the old S-9 on vintage receivers us hams remember. 50 microvolts to the input = S-9. 34 dBµ also = 50 microvolts on these radios. A respectable signal. 80 dBµ is huge and it's not surprising this can cause overload.

Also, consider that the induced voltage in a loop increases linearly with the number of turns, the area of the loop, and the frequency. An 8 inch loop has nearly twice the voltage output as a 6 inch loop. Area 64/36 = 1.77 times the output. An 18 inch loop has more than twice the voltage output of a 12 inch loop. Area 324/144 = 2.25 times the output.

Here's a curious tuning tip that may not be apparent at first. It requires two DSP radios. Hard-wire a square loop to the first DSP radio (ferrite removed). This DSP radio tunes the loop in place of a 365 pF variable capacitor if you don't have a spare, sort of a "digitally tuned passive loop". Couple this loop inductively to the second, unmodified DSP radio like you normally would. Now you can accurately tune the passive loop by reading the frequency on the first DSP radio. Overkill, yes. But an interesting hack.

Bottom line on building a very small loop - 8 inches would be a minimal size for any appreciable gain.

FURTHER THOUGHTS ON HARD-WIRED vs. PASSIVE-TUNED LOOPS

I was all-in for awhile with the hard-wired loop, connected directly to the DSP's front end. The DSP chip tunes the coil just like the ferrite. What more could you want?

The problem here, again, is potential overload. These little DSP radios are not bullet-proof like the table-tops of the old days. An 18 inch or larger hard-wired loop produces massive un-tuned signal to the front end of the radio across the entire mediumwave band. The key word here is "un-tuned". You are relying solely on the DSP's existing circuitry to differentiate massive signals across a wide range of spectrum. Hard-wired loops above a certain size invite overload. We seem to crossover into this territory above the 12 inch range.

Hard-wired loops are great in quiet locations, especially for long distance daytime DX. Nighttime is a different story, as signal strengths can equal urban levels even without the loop-assist. You must find a way to incorporate selectivity to reject or lessen that which you are not interested in.

To summarize, using a passive tuned-loop and coupling inductively gives you two additional important advantages over the hard-wired loop:

1. A double-tuned front end. A tuned-loop has selectivity itself and its passband will essentially only present that large signal on the channel to which it is tuned.

2. The ability to decouple the loop from the radio by moving it farther away is sort of a poor man's RF gain control. If the radio is still overloading, move the loop away a bit.

An additional option to the passive-tuned loop is the successful FSL or Ferrite Sleeve Loop antenna. It is compact. If tuned with a variable capacitor, it will reject off-channel signals, lessening front-end overload as it serves as a double-tuned front end again. And it is directional of course. The C.Crane Twin Coil 200mm ferrite has similar qualities, a double-wound ferrite loop with a signal peaking ability.

ABOUT LOOPS AND NULLING

This applies not only to open air loops but to ferrite loopsticks as well. Rotating the loop off the station "peak" varies the induced signal voltage according to the trigonometric sine of the angle away from the peak. At 30 degrees off peak you have reduced the signal pickup by half the voltage (50%). At 45 degrees, 70% reduction. At 60 degrees, 86% reduction. At 90 degrees, theoretically 100% reduction, or zero signal. Now, we all know that 100% reduction is generally not theoretically obtainable, but sharp loops do come close.


The image above depicts the signal pickup for various angles of the signal arrival in relation to the side of the ferrite loopstick. Note that at only 30 degrees off the zero null the signal has increased to 50% of its full value. Passive box-type loops react exactly the opposite. Full signal occurs end-on to the loop instead of broadside.

Closed loops have magnetic pickup by nature. They receive the magnetic component of the passing wave. The magnetic component of a mediumwave station's signal is horizontal and parallel to the ground. Its electrical component is in the vertical plane. This is why when you rotate a radio's ferrite loopstick towards vertical the signal level drops to near zero.

WINDING FERRITE RODS AND BARS

Paul S., in Connecticut, who participates in the UltralighDX group, has some great tips on winding the ferrite coil:

The older and later 'pocket radio' designs were both made with a mix of price and quality. Older designs had weaker transistors/tubes and needed a high quality ferrite coil winding. A pocket model of the 60's had better transistors and could be made physically smaller, the circuit would compensate for the smaller ferrite rod.

Nonetheless, both designs are not optimum, as the most recent studies have shown. I refer to Ben Tounge's (of Blonder-Tounge Radio/TV) Article #29 at his namesakes website. I also note that there were mathematical errors in popularly published catalogs of ferrite material.

So what is a near optimum design one might ask?

1.) Keep the ferrite bar at least 10 times its diameter for nulling properties.

2.) Try to keep the coil winding between 1/2 and 1/3 of the ferrite bar length.

3.) Space the coil winding 3x the wire diameter (in typical designs its usually 2x the diameter). For example #30 gauge wire of 0.01" diameter is wound at 0.03" instead of the typical 0.02". This removes some "proximity effect" especially when many turns are involved.

4.) The wire diameter itself has to be minimized to reduce the "Skin Effect". This seems counter-intuitive because thin wire has more resistance per foot (meter). But, that's DC resistance... we are looking to reduce AC (AM radio signal) resistance. Such AC resistance gets larger when very little current flows in the center of the wire: the smaller diameter wire "saturates" the wire better, with more AM radio signal current flowing in the center of the wire.

5.) Because of #3 and #4 above, a single wire of small gauge (say #30 or better yet #32) can be wound upon the ferrite bar. However, the best choice is still Litz wire based upon #46 gauge strands. Its more expensive, and you do get what you pay for. I will also say that one would be surprised at how good a single #32 wire (0.008") spaced at 0.024" is.

6.) Using a single wire, one can wind directly on the ferrite bar (most of this 'magnet wire' is insulated), but most prefer using a layer of heat-shrink tubing over the ferrite bar.

So, in recap, the older radios got the wide spacing right, and the newer ones got the small gauge wire right, but neither got both right. One thing not mentioned in most commercial radio designs is the actual inductance of this ferrite bar antenna. I do know that some designs are intentionally high up to 700µH for Sony's in the 60's-70's. Modern DSP designs need 350-500uH, and can use short bar lengths (re: small wire diameter).

Regards,

Paul S. in CT

Thanks for the tips, Paul.

WIRE ANTENNAS

Loops in general are directional and have many advantages over simple wire antennas, should you have a radio which you can attach a wire antenna to. The non-DSP Sangean ATS-909X, the C.Crane EP Radio Pro, and Tecsun PL-880 are three which allow direct connection of wire antennas.

I find directly-connected (or inductively-coupled) horizontal short wire antennas for either DSP or non-DSP radios to be somewhat useless on the mediumwave band. Note, I am talking about SHORT wire antennas. Longer wires, and the longer the better, are preferred to the short variety. By longer, we are talking 200 feet or better. By short, I generally mean about 50 feet or less.

The Beverage antenna or "wave antenna" is a very long wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It's routinely run about six feet off the ground and may or may not be terminated at the far end with a resistor directly to ground. It has massive signal gathering ability and is also somewhat directional. A variety of Beverage, the BOG antenna (Beverage On Ground) has the same attributes going for it, laid directly on the ground. Neither are generally tuned, but can be. Beverage antennas work best if their length approaches a full wave or greater.

Another option is the flag or otherwise large-sized, single wire rectangular loop antenna. It may be 20 ft. tall by 50 feet long or larger. It is directional, and characteristically very quiet as loops are. It can be tuned as well. It has excellent performance.

THE VERTICAL ANTENNA

Lastly, a different option than the rest is the vertical antenna, an antenna poo-pooed by many but one which I've had many successes throughout a 55 year radio hobby. As a ham, over my active years I used verticals on the HF bands to work stations world-wide using only flea power at times. Using a simple vertical, I have worked long-path propagation from Denver to Sweden over the Pacific and Indian Oceans using CW (Morse) for example, a distance of some 19,000 miles. Their low angle take off is exceptional. But what about mediumwave?

Believe it or not, they are also useful for mediumwave reception. After all, the vertical is used by mediumwave stations to transmit!

The key to the vertical in all situations is matching it properly through a balun transformer of some kind. This will enable maximum signal (voltage) transfer to the receiver. It doesn't have to be tuned, just matched. I use a 25 ft. vertical here, matched through an RF Systems Magnetic balun, essentially a 9:1 transformer. 50 feet of coax is run from the RF Systems balun to the inside of the house. Coupling to the receiver is done inductively. Wind 15-20 turns of insulated wire around a 4 inch ferrite bar and connect to the coax, one side to center and one side to ground. Position the 4 inch ferrite near the radio's internal ferrite. The resultant increase in signal pickup is astounding.

A 25 foot vertical seems short for the mediumwaves, but its signal output is as good or better than a 48 inch passive-tuned loop. If there is a drawback it's that it is omni-directional versus the passive loop. However this can be advantageous for certain types of bandscanning. Park on a frequency for several hours while you are doing something else and you will hear the fade up and down of several stations. You will get to know the relative signal strengths of each over the long term listening period.

Also going for it is its same-plane alignment to the transmitted signal (vertical). The short horizontal wire antenna is 90 degrees to the mediumwave signal's polarization plane and perhaps even 180 degrees end-on or 90 degrees broadside to the station as well. A 20 dB or greater difference in signal strength can be realized in the vertical over the horizontal wire antenna. 20 dB is an increase of more than three times the signal pickup voltage.

Note that groundwave reception, even over extreme distances during daytime hours, is virtually in the vertical plane at all times. Nighttime skywave varies, but hardly less than 30 degrees off of vertical even at extreme distances. You can prove this yourself using your portable radio with internal ferrite loop antenna. At night, tune to a medium to strong distant station. Tilt the radio from horizontal towards vertical and the signal will be reduced dramatically as you approach the 45 degree point. The effect will be less-pronounced for stations closer than about 300 miles which have a higher angle arrival.

Much mediumwave fun can be had through experimenting with different antennas. Give it a try.

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